Treatment of solid containing material derived from effluent

ABSTRACT

A method of treating solid containing material derived from effluent or sludge from a plant for de-inking paper, the material containing calcium in the form of one or more insoluble calcium compounds, the method including the steps of treating the material with an acid to cause dissolution of the calcium thereby forming a calcium ion-containing solution in which insoluble solids are suspended, separating the solution from the insoluble solids and incinerating the separated solids. The solution containing calcium ions may be treated by adding one or more reagents to form a calcium compound precipitate, eg calcium carbonate. The particulate solids produced following the incineration step and following the precipitate formation may be employed as pigments or fillers in paper making or paper coating.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to the treatment of solid containingmaterial derived from effluent, especially to recover useful materialstherefrom.

In particular, this invention is concerned with a method for recoveringuseful materials from what has hitherto been regarded as the wastematerial produced by an industrial plant for treating paper, especiallyfor de-inking waste paper.

Paper and paperboard products are generally made by preparing a diluteaqueous suspension containing cellulosic fibres derived from wood orfrom some other suitable fibrous material, and de-watering thesuspension on a moving wire mesh belt to form a sheet material. Thesuspension may also contain a proportion of a mineral filler materialwhich improves the brightness and opacity of the finished paper orboard. Natural cellulosic fibrous materials are generally expensive, andthe trees or other plants from which they are obtained must be replacedif the balance of the environment is not to be disturbed. It istherefore desirable to incorporate in the aqueous suspension from whichthe sheet material is made a substantial proportion of cellulosicfibrous material which has been recovered from waste paper. Most wastepaper which is recycled has been printed on at least one of itssurfaces, and it is necessary to separate the printing ink residue fromthe cellulosic fibres if the recovered material is to be suitable foruse as a starting material in a paper making process. If the ink is notseparated, the fibrous material will generally be unacceptably dark incolour. Generally, printing ink contains elemental carbon (carbon black)and other dark material which causes the dark coloration.

Commercially operated processes for de-inking waste paper generallycomprise a pulping stage in which the waste paper is subjected tomechanical agitation in water which also contains sodium hydroxide whichcauses the fibres to swell, and which breaks down most ink vehicles bysaponification or hydrolysis, sodium silicate which acts as a pH bufferand agglomerates detached ink particles to a convenient size, and asurfactant which wets the ink particles and helps to keep them insuspension. The suspension formed in the pulper is passed through aprimary screening system in which heavy foreign bodies, such as staples,paper clips and pieces of grit are removed. The suspension passingthrough the screens is then fed to a treatment plant comprising one ormore froth flotation cells, or one or more washing units, or acombination of washing units and flotation cells. The froth flotationcells are each provided near the bottom with a rotating impeller andmeans for admitting air under pressure in the form of fine bubbles inthe region of the impeller. It may also be advantageous to add to thesuspension, before it enters the flotation cell, a reagent known as acollector which attaches itself preferentially to the ink particles andincreases their affinity with air relative to that with water. As aresult, the ink particles are preferentially lifted by the air bubblesto the surface of the suspension in the flotation cell, where they aredischarged in a froth product.

In the washing units the pulp is subjected to agitation in fresh orrecycled water, optionally containing a dispersing agent, and the washedpulp is then drained on a screen of aperture size such as to retain therelatively long cellulosic fibres, while allowing to pass the mineraland organic particles and the fine fibre fragments.

The suspension which is discharged from the bottom of the flotationcell, and/or the suspension which passes the screens of the washingsteps, is de-watered and the de-watered material, which consists ofsubstantially de-inked cellulosic fibre material, may be subjected tofurther purification steps before being finally de-watered and dried forre-use in a sheet forming process.

The de-inking treatment plant reject consists of an aqueous effluent orsludge which is produced in large volumes. The reject from some plantsis discarded and transferred to a waste disposal site, eg a landfillsite. The reject from other plants is incinerated and this beneficiallyreduces the volume of the reject and generates useful heat energy fromthe combustion of organic components. The ash from the incineration issubsequently disposed of. However, in both cases, there is anundesirable cost and environmental impact associated with disposal.Also, potentially useful materials are wasted by the disposal process.The de-inking treatment plant reject generally contains, in addition tothe ink particles and various additives employed in paper, a substantialproportion of the inorganic filler particles which were originallypresent in the waste paper. These filler particles usually consistpredominantly of a mixture of kaolin clay and calcium carbonate invarious proportions, although other inorganic filler particles such assilicates, talc, calcium sulphate or titanium dioxide may also bepresent in minor proportions.

RELATED PRIOR ART

In the prior art, various patent specifications have been publishedwhich describe processes for incinerating waste solids from de-inkingsludge. In some of these processes the ash produced followingincineration is further treated to render the particulate materialcontained in the ash suitable for re-use in a paper making process.Examples of specifications describing such processes include WO96/32354.Where the sludge solids treated include calcium carbonate this can reactwith kaolin present to produce a glassy composite phase which causes theparticulate product to be abrasive. As described in EP798268-A1, theincineration temperature may be controlled so as to minimise formationof the glassy phase, but this procedure can be relatively complicatedand expensive to operate and may cause the resulting particulate productto have inferior optical properties, eg a reduced brightness.

SUMMARY OF THE INVENTION

The present invention allows treatment of calcium compound-containingsolids obtained from a waste paper de-inking sludge to be carried out ina manner which does not suffer from the problems of the prior publishedprocesses referred to earlier and allows improved products suitable forre-use as pigments and fillers, eg for addition to papermakingcompositions, to be obtained in a simpler and cheaper manner.

According to the present invention there is provided a method oftreating solid containing material derived from effluent or sludge froma plant for de-inking paper, the material containing calcium in the formof one or more insoluble calcium compounds, the method including thesteps of treating the material to cause dissolution of the calciumthereby forming a calcium ion-containing solution in which insolublesolids are suspended, separating the solution from the insoluble solidsand incinerating the insoluble solids.

The treated solid containing material may comprise the said effluent orsludge, optionally after diluting with water, and may comprise theparticulate materials described hereinbefore. The said calcium compoundmay comprise calcium carbonate and/or calcium sulfate and/or calciumsilicate, and/or calcium phosphate.

The method of the invention may surprisingly and beneficially beoperated in a less costly manner than that described in EP798268-A1,because it does not require use of the close temperature control (whichrequires use of expensive automation equipment), multiple calcinationsteps and recarbonation (following aqueous dispersion) of the resultingash all as required in the process of EP798268-A1. As illustrated later,the product of the method of the invention, even though produced by aless complicated and less costly process, shows properties relevant tore-use, eg in a paper making process, comparable with those obtained forthe product of the process of EP798268-A1.

DESCRIPTION OF THE INVENTION

The said treatment to cause dissolution of the calcium comprisesaddition of an acid which may be a dilute acid. The dilute acid used maycomprise any one or more of the dilute acids known to form solublecalcium salts. Mineral acids such as hydrochloric acid are preferred.Some dilute acids, eg sulfuric acid, which may be employed will causethe calcium ions released to form a partially soluble salt, eg calciumsulfate. In this case, the partially soluble salt will remain in thesubsequently separated and calcined solids.

In the method according to the present invention the calcium dissolutionstep may be carried out with the assistance of heating and othersuitable agents, eg mechanical agitation or stirring.

After calcium has been dissolved over a sufficient period of time, whichwill depend upon the amount of calcium to be dissolved, the calcium ion-containing solution will generally have suspended therein particulatesolid material whose content will depend on the composition of theoriginal de-inking effluent or sludge and any treatment which has beenapplied to it. The solid particulate material may be separated from thecalcium ion-containing solution by one or more suitable known separationtechniques, eg filtration, evaporation or by use of a hydrocyclone or acentrifuge, applied in one or more stages.

The solid material remaining after the acid dissolution step may betreated, optionally after a drying step, by one or more heat treatmentsteps which provide incineration of the combustible organic compoundspresent and calcining of the remaining particulate material which islikely to comprise principally kaolin.

The heating step applied to calcine the kaolin-containing particulatematerial may be carried out at a temperature higher than that employedin the examples described in EP798268-A1. For example, a singleincineration and calcination step may be applied in a known incinerationdevice, eg a fluidised bed furnace, at a temperature of between 900° C.and 1200° C., desirably from 950° C. to 1050° C. Carbon present from theink contained in the de-inking sludge is oxidised quickly at thistemperature.

The resulting particulate material following incineration andcalcination may be further treated, either in a dry form or in the formof an aqueous suspension, to improve the properties of the particulatematerial, eg by comminution such as attrition or media grinding in aknown manner, particle size classification (if not previously applied),washing, addition of chemical agents, eg optical brightening agents,coagulants etc. The resulting particulate product may be delivered indry or aqueous slurry form to a paper mill for use in a paper making orpaper coating process in a known manner optionally together with otherknown pigments, eg kaolin, calcined kaolin, calcium carbonate, talc,titanium dioxide, talc or calcium sulfate.

Where the recovered product is recycled for re-use in a paper mill, thepaper mill may be associated with the de-inking plant from which thetreated sludge has been obtained.

Where the recovered product is recycled for re-use in a paper mill itmay be used as a paper making filler in the mill in a known manner. Thisfiller, optionally mixed with other fillers as described earlier, may beemployed as a dilute slurry by mixture with a dilute cellulose fiberslurry to produce a paper making furnish. The solids of the furnishcomposition may for example constitute from 0.2% to 2%, eg from about0.5% to 1% by weight of an aqueous suspension.

After the solid material has been separated the calcium ion-containingsolution may be treated to form a precipitate of useful insolublecalcium compound. An alkali metal salt, eg a sodium salt, may be addedto the solution to provide the required anion. For example, sodiumcarbonate may be added to the calcium ion-containing solution to producea calcium carbonate precipitate. Where the calcium ion-containingsolution comprises calcium chloride, sodium chloride is left insolution.

Alternatively, an alkali metal hydroxide, eg sodium hydroxide, may beadded to release calcium hydroxide.

Where the calcium ion-containing solution contains calcium hydroxide,formed for example by addition of an alkali metal hydroxide to a calciumsalt solution, carbon dioxide may conveniently be added to the calciumhydroxide solution to form a calcium carbonate precipitate. Formation ofprecipitated calcium carbonate by addition of carbon dioxide to calciumhydroxide is, of course, well known in the pigment producing art.

The insoluble calcium compound product thereby formed, eg calciumcarbonate, may be separated from the aqueous medium in which it isformed by a suitable separation process, eg filtration, evaporation orby use of a hydrocyclone or a centrifuge. The product may be stored andthereafter supplied for use in one of the applications describedhereinafter, especially as a particulate filler in the manufacture orcoating of paper. Where the solution contains little dissolved materialafter formation of the calcium compound precipitate, eg after calciumhydroxide has been converted to calcium carbonate using carbon dioxide,the suspension or slurry containing the precipitate may itself be usedoptionally with partial dewatering.

For example, the suspension may be delivered to a paper mill for use ofthe suspension as a particulate filler supply for use in papermanufacture or one of the other pigment or filler applications describedfor the calcined kaolin-containing material described earlier. Themethod of use may be as for that material.

If the solution in which the insoluble calcium precipitate is producedcontains dissolved species these may optionally be recovered andre-used. For example, where the dissolved species comprises sodiumchloride solution this may be electrolysed in a known way to producesodium hydroxide plus hydrogen and chlorine gases. The gases may berecombined to produce hydrogen chloride from which hydrochloric acid canbe recovered which can be re-cycled for re-use in the acid dissolutionstep. The sodium hydroxide can be recycled for re-use as the alkalimetal hydroxide for addition to the calcium ion-containing solution. Inaddition, carbon dioxide is produced in the dissolution step this may bere-cycled for re-use in the precipitation step. In this way, the need topurchase chemical reagents and to dispose of by-products can beminimised or avoided.

The method according to the present invention beneficially allowsminimization of the amount of solid waste from a de-inking plant therebyreducing or eliminating the cost and environmental impact of disposal ofsuch waste. Furthermore, the method allows one or more useful productsto be recovered in a manner which is simpler and cheaper than therelevant prior art.

The useful product recovered includes a calcined kaolin-containingparticulate material which does not suffer substantially from theabrasiveness normally caused by incinerating kaolin and calciumcarbonate together. In this case, the calcined kaolin composition may beused in various pigment employing applications, eg paper making asdescribed earlier or in paper coating, filling of polymers, paints andthe like. Such a product may show improved optical properties, egbrightness, compared with products produced by incineration of solidsfrom de-inking sludge by prior published methods without discolorationfrom ink coloring materials, eg carbon, and in a cheaper manner.

A further product, for example calcium carbonate, may also be recoveredwithout the dark coloration normally associated with de-inking rejectand is suitable for use in the various applications in whichprecipitated calcium compounds, eg calcium carbonate, are known to beuseful, eg as a particulate filler or extender material in paper,polymers, paints and the like or as a coating pigment or colouringredient for coating of paper, paper board, plastic papers and thelike.

Other particulate materials, eg silicate and aluminosilicate materialcontained in the de-inking reject may, as described hereinbefore, beconverted after calcination into useful products, eg pozzolans, and thereagents used in the method according to the present invention may berecycled thereby minimizing reagent costs and waste disposal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic flow diagrams of alternative methodsembodying the invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings.

As shown in FIG. 1, in a first example of a method embodying theinvention, input solid-containing material 1, preferably in the form ofan aqueous suspension, comprises, reject sludge from a paper de-inkingplant (not shown).

The material 1 is subject to an acid dissolution step in a vessel 3 inwhich calcium present in the input material is dissolved as a solublesalt, eg calcium chloride, by use of dilute hydrochloric acid. Thesuspension formed by the acid dissolution step is subject to asolid/liquid separation step in a separator 5. Solids obtained by theseparation step in the separator 5 is delivered to an incinerator 9, egoperated at a temperature of 800°C. to 1200° C. as appropriate, whichoxidises the combustible organic material and carbon present in thesolids and produces an inorganic product 11 comprising predominantlycalcined kaolin (metakaolin). The product 11 optionally with furthertreatment, eg comminution, particle size classification and washing, maybe delivered in dry or slurry form for use as a filler in paper makingin the manner described earlier.

The solution separated by the liquid/solid separation step in theseparator 5 may be delivered to a vessel 12 in which sodium carbonate isadded in a quantity calculated to give a required amount of precipitatedcalcium carbonate according to the amount of calcium present. Thesolution entering the vessel 12 may be monitored on-line by a monitor 13to determine its calcium ion concentration and hence the requiredconcentration of sodium carbonate in the vessel 12. The solutionseparated by the liquid/solid separation in the separator 5 mayalternatively be delivered to a vessel 15 via a line also monitored by amonitor 13 in which sodium hydroxide solution is added to the solutionto form calcium hydroxide which will dissolve in the water present up toits limiting solubility. Carbon dioxide is added in a reactor 17 (whichmay be the same as the vessel 15 or a separate reactor). The requiredamount of carbon dioxide to be added may, as an indication, becalculated from the amount of calcium hydroxide present which in turncan be determined from the concentration of calcium ions monitored bythe monitor 13. It is not essential to calculate the required amount ofcarbon dioxide since after all calcium hydroxide has been consumed thepAd of the solution will drop indicating that the reaction hasterminated.

The precipitate-containing slurry formed either in the vessel 11 or inthe reactor 17 is subjected to dewatering, eg by evaporation, in aconcentrator 19. Output material 21 in slurry or dry powder form, asappropriate, from the concentrator 19 is delivered to a storage tank 23for onward supply to a user plant, eg a paper making mill or plant inwhich the material 21 is to be used as a paper filler material in themanner described earlier.

Liquid 22 separated by the concentrator 19 may be discharged as effluentor may be treated to recover reagents therefrom. For example, where theliquid 22 comprises predominantly sodium chloride solution, this may beelectrolysed to produce hydrogen and chlorine and to leave sodiumhydroxide. The hydrogen and chlorine may be combined to produce HCl forre-use in the acid dissolution step in the vessel 3 leaving, from theelectrolysis, sodium hydroxide in the vessel 15 which may be re-used.

An alternative method embodying the present invention is illustrated inFIG. 2. In this case, input material 25 comprises de-inking planteffluent sludge preferably diluted with clean water. This material 25 isdelivered to an acid tolerant vessel 27 in which dilute hydrochloricacid is added. The suspension remaining in the vessel 27 is subjected toliquid/solid separation in a separator 29. The solid separated may bewashed in a washer unit 31 and thereafter dried in a dryer 26 andincinerated in a fluid bed incinerator 28 operating at a temperature ofabout 1000° C. Heat energy 36 is generated by the combustion of organicmaterial in the incinerator 28. Carbon is oxidized in the incinerator.The solids output or ash from the incinerator 28 indicated by output 34comprises predominantly calcined clay (metakaolin). This product may beused as a pigment or filler as described earlier, eg by re-use in papermaking.

The solution indicated as 30 separated by the separator 29 and/or by thewasher unit 31 is delivered to a concentrator 33. Extracted water 32 maybe re-used in the washing unit 31. The solution concentrated by theconcentrator 33 comprising mainly calcium chloride is delivered to anelectrolytic cell 35. Hydrogen 37 and chlorine 39 are produced by thecell 35 and are delivered to a reactor 41 in which they are reacted toform hydrogen chloride gas 42. The HCl gas 42 may be recycled afterdilution as dilute hydrochloric acid for addition in the vessel 27.

A calcium hydroxide-containing slurry formed by electrolysis of calciumchloride in the cell 35 is delivered to a reactor 43, in a meteredamount, and an appropriate amount of carbon dioxide 45 which may beobtained as a by-product from the acid dissolution in the vessel 27 issupplied to the reactor 43 to provide conversion of the calciumhydroxide present to precipitated calcium carbonate. The product slurryformed may be concentrated by dewatering in a concentrator 46 anddelivered to a storage tank 47 for onward delivery as a product, eg as aparticulate filler in paper manufacture. Alternatively, the productslurry formed in the reactor 43 may be delivered via the storage tank 47for use without dewatering. Water 46 obtained from the dewateringprocess in the concentrator 46 may be recycled for dilution of the HCloutput 42.

EXAMPLE

A deinking sludge having a calcium carbonate content of about 5% byweight and a kaolin content of about 40% by weight, based on the drysolids weight of the sludge, was treated in the manner described withreference to FIG. 1 by addition of dilute hydrochloric acid to dissolvethe calcium carbonate present followed by incineration and calcinationin a furnace at a temperature of about 1000° C. for several hoursovernight. The product obtained was redispersed in water and milled bysandgrinding and then subjected to the tests described later in thisExample. The Product is there referred to as ‘Product 2’. For comparisonpurposes, a further product, Product 1, was made without aciddissolution by the more complicated and costly method described inEP798268 using carefully controlled incineration and calcination at atemperature of 750° C.

Products 1 and 2 were tested as follows:

The ISO brightness was measured and the particle size distribution wasmeasured using a SEDIGRAPH 5100 machine. The results which were obtainedare as shown in Table 1 as follows.

TABLE 1 PHYSICAL PROPERTIES Product 1 (comparative) Product 2Brightness, ISO   82.2   83.4 PSD    % <10 microns 100  100     % <5microns 100  100     % <2 microns 97 99   % <1 micron 88 92  % <0.5micron 53 64 % <0.25 micron 12 24

These results show that a better brightness and finer particle sizedistribution are obtainable using a method embodying the invention whichis simpler and cheaper to run than that described in EP798268-A1.

Products 1 and 2 were evaluated in 48gsm newsprint to compare theirperformance as fillers.

Handsheets were made on a TAPPI sheet mold and dried on a Noble & Wooddrum dryer. The sheets were then conditioned before testing.Experimental conditions for the handsheet study are given in Table 2below.

The handsheets were tested for brightness, opacity, caliper, Gurleyporosity, burst and % ash. Scattering and absorption coefficients alongwith bulk were calculated. The handsheets were then calendered,conditioned again and tested.

Handsheet properties are given in Table 3 below. Sheet properties wereinterpolated at 5% filler to give a comparison of the pigments. Theresults obtained are shown in Table 4 below.

TABLE 2 EXPERIMENTAL CONDITIONS OF SHEET FORMATION The followinghandsheet preparation conditions were employed to make handsheets inaccordance with TAPPI standard T205.om-88 (“Forming handsheets forphysical tests of pulp”) Pulp 90% TMP 10% Kraft Final consistency 0.3%solids in aqueous suspension Grammage 48 gsm pH 4.8 Sheet Mold TappiCalendering conditions Machine calender at 38° C. 1 nip @ 150 psi 1 nip@ 250 psi 1 nip @ 250 psi

For calendering preparation, the handsheets were placed in thedesiccator for 10 minutes with water to allow the moisture content ofthe sheets to increase to ˜8%. Between each nip the sheets were placedback in the desiccator for 5 minutes and then only one set was removedat a time.

TABLE 3 UNCALENDERED HANDSHEET PROPERTIES ISO % Grammage BrightnessPigment Tappi Hunter Optest Gurley Burst gsm Ash % Scatter Opacity L a bFormation Porosity Index Product 1 Sample 1 47.0 2.8 59.3 2652 88.2 87.0−1.3 14.0 90.4 13.6 1.73 Sample 2 46.4 6.0 61.6 3164 90.3 87.9 −1.3 13.284.1 15.5 1.75 Sample 3 45.7 8.3 61.6 2485 90.6 87.6 −1.3 12.9 76.1  8.41.81 Product 2 Sample 1 46.6 2.9 59.4 1995 87.5 87.1 −1.3 14.0 94.1 12.81.82 Sample 2 46.8 6.0 61.6 2414 89.0 88.0 −1.3 13.4 88.1 12.1 1.53Sample 3 45.7 8.7 62.7 2236 89.6 88.4 −1.3 13.0 83.9 11.5 1.49

TABLE 4 CALENDERED HANDSHEET PROPERTIES ISO DATA % Grammage BrightnessPigment Tappi Hunter Gurley filler gsm Ash % Scatter Opacity L a bPorosity Product 1 Sample 1 47.0 2.8 58.9 2402 88.3 86.5 −1.2 13.7 37.1Sample 2 46.4 6.0 61.1 2731 90.0 87.4 −1.1 13.0 35.4 Sample 3 45.7 8.361.3 2400 90.7 87.2 −1.1 12.6 29.4 Product 2 Sample 1 46.6 2.9 59.3 182287.6 86.8 −1.1 13.8 42.2 Sample 2 46.8 6.0 61.3 2014 88.6 87.6 −1.1 13.227.9 Sample 3 45.7 8.7 62.3 2202 89.9 88.0 −1.1 12.7 26.5

The results shown in Tables 3 and 4 illustrate that the product obtainedby the method embodying the invention gives similar properties (whichare superior in some cases) to those obtained for the product of theprocess described in EP798268-A1 but, as described earlier, the productof the method embodying the invention may be obtained by a lessexpensive de-inking sludge treatment process.

I claim:
 1. A method of treating solid-containing material derived fromeffluent or sludge from a plant for de-inking paper, wherein saidsolid-containing material contains calcium in the form of one or moreinsoluble calcium compounds, said method comprising: (A) treating saidsolid-containing material with an acid to cause dissolution of thecalcium, thereby forming a calcium ion-containing solution in whichinsoluble solids are suspended; (B) separating the suspended insolublesolids from the calcium ion- containing solution in (A); and (C)incinerating the insoluble solids obtained in (B), and wherein fibrousmaterial in said solid-containing material derived from effluent orsludge is not subjected to combustion or ignition prior to (A).
 2. Amethod as claimed in claim 1 and wherein the solid-containing materialincludes kaolin which is calcined by heating during the incineration ofthe insoluble solids.
 3. A method as claimed in claim 1 and wherein theproduct of the incineration, optionally after one or more furthertreatments, is supplied for use as a pigment or filler in a paper makingcomposition.
 4. A method as claimed in claim 2 and wherein the productof the incineration, optionally after one or more further treatments, issupplied for use as a pigment or filler in a paper making composition.5. A method as claimed in claim 1 and wherein the effluent or sludge isdiluted with water prior to the treatment with acid and is dewatered ordried prior to the incineration.
 6. A method as claimed in claim 1 andwherein the calcium present in the solid containing material is presentas one or more compounds selected from calcium carbonate, calciumsulfate, calcium silicate and calcium phosphate.
 7. A method as claimedin claim 1 and wherein the acid treatment to cause dissolution includesadding a dilute acid selected from hydrochloric acid and sulfuric acid.8. A method as claimed in claim 1 and wherein sodium hydroxide is addedto the solution separated by the separation step to form calciumhydroxide and then carbon dioxide is added to the calcium hydroxideformed to form a calcium carbonate precipitate.
 9. A method as claimedin claim 1 and wherein sodium carbonate is added to the solutionseparated by the separation step to form a calcium carbonateprecipitate.
 10. A method of treating solid-containing material derivedfrom effluent or sludge from a plant for de-inking paper, wherein saidsolid-containing material contains (a) calcium in the form of one ormore insoluble calcium compounds, and (b) fibrous material, said methodcomprising: (A) treating said solid-containing material containing (a)calcium in the form of one or more insoluble calcium compounds and (b)fibrous material with an acid to cause dissolution of the calcium,thereby forming a calcium ion-containing solution in which insolublesolids are suspended; (B) separating the suspended insoluble solids fromthe calcium ion-containing solution in (A); and (C) incinerating theinsoluble solids obtained in (B).
 11. A method of treatingsolid-containing material derived from effluent or sludge from a plantfor de-inking paper, wherein said solid-containing material containscalcium in the form of one or more insoluble calcium compounds, saidmethod comprising: (A) treating said solid-containing material with anacid to cause dissolution of the calcium, thereby forming a calciumion-containing solution in which insoluble solids are suspended; (B)separating the suspended insoluble solids from the calciumion-containing solution in (A); and (C) incinerating the insolublesolids obtained in (B).
 12. A method of treating solid-containingmaterial derived from effluent or sludge from a plant for de-inkingpaper, the material containing (a) calcium in the form of one or moreinsoluble calcium compounds and (b) fibrous material, the methodincluding the steps of treating the material containing (a) calcium inthe form of one or more insoluble calcium compounds and (b) fibrousmaterial with an acid to cause dissolution of the calcium therebyforming a calcium ion-containing solution in which insoluble solids aresuspended, separating the solution from the insoluble solids andincinerating the insoluble solids.